1. Field of the Invention
This invention relates to the preparation of polyurethanes and to polyurethanes so prepared and is more particularly concerned with polyurethanes prepared from an organic polyisocyanate, an aliphatic glycol extender and a particular mixture of two or more polyether polyols.
2. Description of the Prior Art
The preparation of polyurethanes by reaction of an organic polyisocyanate, a polyether polyol and a low molecular weight extender such as an aliphatic glycol is commonplace in the art. The use of copolymers of ethylene oxide and propylene oxide, having varying functionality depending upon the functionality of the initiator used in the copolymerization, has been found to confer particularly useful structural strength properties on such polyurethanes both in cellular and non-cellular forms. Illustratively, U.S. Pat. No. 3,857,800 shows the preparation of polyurethane foams with a reduced tendency to shrink by use of a combination of two polyols one of which can be an ethylene oxide-capped polyoxypropylene polyol having a molecular weight of 3000 to 6000 and a primary hydroxyl content of 20 to 70% and the other of which can be an ethylene oxide-propylene oxide copolymer having a molecular weight of 500 to 2000 and an ethylene oxide content of 20 to 80% by weight. No aliphatic glycol extender is employed in preparing the foams disclosed in this reference.
U.S. Pat. No. 3,945,939 discloses a problem which exists in the preparation of polyurethanes by reaction of an organic polyisocyanate with an ethylene oxide-capped polyoxypropylene using ethylene glycol and like low-molecular weight glycols as the extender. The low molecular weight glycol is incompatible with the polyol in the proportions employed and the reference shows that colloidal silica or certain clays could be utilized to assist in forming a stable dispersion of the glycol in the polyol prior to reaction.
U.S. Pat. No. 4,273,884 describes substantially the same problem and overcomes it by forming an emulsion of the polyol and the low molecular weight glycol prior to reaction with the polyisocyanate.
U.S. Pat. No. 3,798,200 describes the preparation of polyurethanes having improved cut growth and flex-crack resistance by reacting an organic polyisocyanate with a mixture of two different polyether polyols and any of a wide variety of active-hydrogen containing extenders which can include low molecular weight glycols. The two polyether polyols were chosen from a wide variety of such polyols the criticality being said to lie in that one of the two had a peak in the high end of the molecular weight distribution curve and the other had its peak in the lower end of the range but the average molecular weight of the mixture of the two polyols was in the range of 4500 to 20,000. U.S. Pat. No. 3,963,681 discloses a closely related development in the same area but calls for the average molecular weight of the mixture of the two polyols to be in the range of 1000 to 4500.
U.S. Pat. No. 4,065,410 describes a method of increasing the green strength of polyurethane elastomers having a compact skin and a cellular core by using as the extender a mixture of ethylene glycol and up to 30 percent by weight, based on total weight of extender, of a polyol having a molecular weight of less than 1800. A wide variety of the latter polyols are disclosed. Compatibility of the ethylene glycol with the polyol component is not discussed.
We have now found that, not only can the problem of compatibilizing a low molecular weight glycol extender in a polyether polyol be solved in a highly satisfactory manner, but certain properties of the polyurethanes produced from the compatibilized components are greatly enhanced as a direct consequence of the compatibilization. The latter can be achieved, as described below, by the use of a combination of at least two carefully chosen groups of polyoxyethylene polyoxypropylene polyols in proportions such that the aliphatic glycol extender is compatible therewith in the range of proportions in which it is required to be used in preparing the polyurethane.